Shimano 105 10-spd Crank with 9 spd Cassette and 9 spd Chain?
 

Hi - I currently have a Shimano 9 speed setup: 9 speed tiagra shifters and Crank, 9 speed chain and 9 speed cassette. I'm thinking of buying a new Shimano 105 (5600) 10 speed crank as well as the appropriate bottom bracket for this crank but leaving everything else (chain, cassette) at 9 speed. Will I have problem with shifting?

Haven't tried it personally, but I bet it'd work fine. There is a slight difference in spacing between the chainrings on a 9 vs. 10 crankset, but probably negligible.

Should work fine as is. If needed you could shim the rings with thin spacers.

I have tried this combo personally with poor results. The 5600 crankset has pretty narrow spacing between the rings and I would get rubbing in 4 of the 9 smaller cogs in the rear when on the small ring on the front. Go with the 5700 series crankset instead that has more relaxed spacing.

Mixing nine and ten speed components generally doesn't produce optimum results with the exception of rear derailleurs (which don't seem to care how many gears you have). The crankset for a ten speed cogset will work but with a lot of tuning problems. You may notice some hesitation or miscuing when shifting and you will get lots of slippage (tooth grinding) if you don't have your cable set exactly right for the shifter detents. In short, even though is only a tiny difference in width between a nine and ten speed chain, the tolerances are so close for these systems that it is a significant factor when trying to mix components.
I am curious as to why you are changing cranksets? The chainrings for a ten speed system are thinner - meaning not as robust, durable, strong, etc., as a necessity to accomodate the narrower chain. Why would you want this when you don't need it?

I am one of those luddites who believes that any system that involves more than eight cogs is overkill. I have a ten speed system and I find that for most of my shifts I move two cogs. I still occaisionally do a double cog shift on my nine speed system too but not as often.

200k

I am wondering why the change as well but the rest of your quoted post is simply wrong. 10 speed chains are only more narrow on the outside. The inner width is identical to a 9 speed chain. As such, 10 speed chainrings/cogs/jockey wheels are not any thinner. I believe that some Shimano 10 speed cranksets use a slightly more narrow spacing (I believe only for the inner to middle ring but could be remembering incorrectly) and the ramps and pins are optimized for the 10 speed chain. Back when Shimano was just getting started with 10 speed stuff there were plenty 9/10 speed cranksets on the market and I own one of them. I've used it with both 9 and 10 speed systems and it's worked perfectly for both. The biggest component that will affect front shift quality is the front derailler. Don't try to mix and match 9 and 10 speed equipment there as it will have negative effects.